Current methods of making powdered metal parts which require hot forming as an integral step thereof, generally require in sequence (a) cold compacting (briquetting) to a density of about 85%, (b) sintering at a high temperature to further increase the density to about 88%, (c) hot coating of a lubricant on the exterior of said compact, usually carried out at a temperature of about 1800.degree. F., and (d) hot forging or hot forming the coated compact, at a temperature level of about 1800.degree. F. with applied pressure in the range of 75 tons per square inch. The compacts are heated in most commerical sintering operations in furnaces which apply the heat by radiation from glowing heating elements. The rate of radiation absorption is governed considerably by the surface condition of the compacts, particularly the color.
Two important problems arise in connection with the accepted method sequence, the first of which relates to the deficiency in time for heating the compact. The time for heating relatively bright and shiny powder compacts, which have been briquetted, is inordinately long, requiring slow belt speeds for carrying the compacts through a continuous sintering furnace. This is inefficient. Secondly, the compacts require a lubricant to facilitate subsequent hot forging under pressure. Lubricants have been applied in the form of dark coatings, typically graphite, but consistently subsequent to sintering and at an elevated temperature level making the coating of such lubricant extremely difficult. The lubricant coating selected has been active graphite or a mixture thereof. Contamination of the iron compact would occur if the active graphite coating were to be applied prior to sintering.
These past sintering lubricant coatings have been applied at two temperature levels, one where the sintered compact has been allowed to be cooled, then reheated to permit the application of the lubricant at a temperature, preferably about 400.degree. F.; then reheated to a required forging temperature, and subjected to hot forming. The other is to take the compact directly from a sintering operation, coat it at the extremely high temperature as it is received from the sintering furnace, and then directly transfer the compact to a hot forging machine.
Both of these alternative procedures do little to promote heating efficiency and make it easier to apply the lubricant. The present invention proposes that a thin, radiation of heat absorbing, shall be applied to the compact under ambient or cold conditions to serve two purposes: (a) to facilitate and increase the efficiency of heat absorption during sintering, and to provide an inherent lubricating coating which retains its character through the hot forming step.
Graphite coatings have been used heretofore in the powder metallurgy art, but limited to their use as a mold coating or a mold medium for carrying out heating to exclude oxidation. For example, in U.S. Pat. No. 3,305,358 several stucco coating layers are applied to a mold in relatively thick amounts. Powdered materials are placed therein and then the assembly is subjected to impact or pressure within the mold. Graphite is used in the relatively thick stucco layering as a mechanism for eliminating die wear; the graphite is not effective to increase heat absorption by radiation from a surrounding space.
In U.S. Pat. No. 3,853,550, a graphite medium was employed within a mold into which a compacted metal object was placed, the graphite layers being at least 1-2 centimeters thick. The use of the graphite medium was to exclude air while the vessel was placed in an air environment for heating. Heating efficiency is not improved; radiant energy must pass through the metal vessel walls and then through the relatively thick graphite medium before effecting a temperature increase in the powder metal part. The thickness and location of the graphite medium is critical to determining whether it acts as an assist to improve heating efficiency or serves as a detriment to heating efficiency.